1. Field of the Invention
The present invention is directed to an improved, high frequency, high or low power, broadband, quadrature combiner having reduced weight, improved electrical performance and simplified tuning capabilities for use in an advanced development high frequency power amplifier.
2. Description of the Prior Art
The prior art quadrature combiner 10, as shown in FIG. 1, performed satisfactorily, but was complex and inherently heavy due to the eight (8) transformers 30,35, 40, 45, 50, 55, 70, and 75 in its design. Further, the combiner 10 was difficult to tune because the negative ninety degree phase network 80 and the zero degree phase network 85 had to be decoupled and individually tuned.
Also, shown in FIG. 1, are the input signal phase relationships for each transformer and the number of transformers for each function. A first input voltage signal 15 shown as V1, and a second input voltage 20 shown as V2, are provided from two distinct amplifiers. Signals 15 and 20 are equal in amplitude and in phase quadrature. In FIG. 1 they are shown entering the input hybrid 25 which comprises hybrid transformers 30 and 35. Signals labeled 16 and 21 are shown as VR1 and VR2, respectively, and are the vector resultant of input signals 15 and 20. As a result of passing through transformers 30 and 35, VR1 is 45 degrees phase shifted from input voltage 15, (V1) and VR2 is negatively phase shifted 45 degrees from input voltage 20, (V2). Also, the characteristic impedance is decreased by one-half from 50.OMEGA. to 25.OMEGA. by hybrid transformers 30 and 35.
The function, therefore, of the input hybrid 25, is to combine signals 15 and 20, and then split the combination into two equal, single-ended output signals, 16 and 21, driving the two pairs of paralleled impedance matching transformers 40/45 and 50/55. Two transformers, 40/45 and 50/55, in parallel, are required for each of the designated matching transformers 60' and 60" in order to achieve the proper characteristic impedance of the transformer windings. Transformers 40/45 and 50/55 also convert the single ended input signals 16/21 into balanced output signals on lines 41/42 and 51/52 which are then applied to the phase networks 80/85. The function of the phase networks 80 and 85 is to shift the phase of the two input signals, 16 and 21, 90 degrees relative to each other and maintain this 90 degrees phase relationship consistently over the frequency range of at least 2 to 30 MHz close to a four octave range. The signals 90 and 95 coming out of the phase networks 80 and 85 thus have the same phase and amplitude and can be combined in the output hybrid 65, which consists of two transformers, 70 and 75. Differences in the phase and amplitude between signals VR1 and VR2 are dissipated as heat in the "dummy" load resistor 71 having a value of 50.OMEGA..
Therefore, this prior art quadrature combiner 10 as shown in FIG. 1 requires eight (8) transformers, 30, 35, 40, 45, 50, 55, 70, and 75 and two phase networks, 80 and 85. This large number of transformers required in the prior art quadrature combiner 10 resulted in difficult tuning problems. Each of the individual transformers required capacitive tuning at the input and output to compensate for series inductive winding reactance. Capacitors were adjusted so that a 3 pole lowpass filter structure was formed with a bandwidth greater than 30 MHz. In general, the large number of transformers and the tuning required led to a degradation in the quadrature combiner's overall electrical performance.
Another serious problem with the prior art combiner 10 as shown in FIG. 1, is that the phase networks 80 and 85 were integrated in the unit 10 in such a way that they could not be individually tuned. The input hybrid 25 connects each of the phase networks 80 and 85 to both sources. Therefore, the networks 80 and 85 are isolated by only 3 dB. Connections in the prior art quadrature combiner 10 must then be broken and terminated in order to isolate the two phase networks 80 and 85 for tuning. Further, the two phase networks 80 and 85 cannot be directly compared against each other for phase quadrature. To meet system requirements, the phase networks 80 and 85 must be tuned to within plus or minus 2 degrees over the frequency band, 2-30 MHz. To achieve this specification requirements, the networks must be tuned externally and then fine-tuned in the unit 10 using a trial and error process. This results in time-consuming retuning and ineffective operation of the quadrature combiner.
The problem to be solved then is the development of an improved quadrature combiner operable for use in ultra-linear, high frequency, communications transmitters. The power amplifiers use feed forward cancellation to reduce intermodulation distortion. These improved quadrature combiners must be operable to cancel most orders of harmonics generated by the amplifiers, thus achieving the required amplifier linearity, and be further operable for use in signal isolation. In the high frequency transmitter, the quadrature combiner must perform in the output line to absorb power reflected from the antenna back into the transmitter. Power not absorbed and re-reflected towards the antenna disturbs the amplifier distortion cancellation loops and corrupts the amplifier linearity.